Limited Flight Evaluation of the Let L-23 "Super Blanik" Test Report

Gary Aldrich, Jeff Nuccio

Originally published March 1995

[NOTE: The following is a flight test report passed along to us by Gary Aldrich, EAA Chapter 1000 member. It has been reformatted due to the limited amount of space in this newsletter. However, the content remains unchanged and I think you'll like it!--ed]

Let L-23 Super Blanik

Background
Scope
Overall Test Objective
Specific Test Objectives
Test Item Description
Test & Evaluation
Test Center of Gravity Matrix
Test Results
Stall
Spins
Conclusions and Recommendations
References

BACKGROUND

The United States Air Force Academy (USAFA) has purchased a L-23 "Super Blanik" aircraft for use in their soaring program. Though the primary stated purpose of the aircraft is for high altitude "wave" soaring, they also desire to shift a portion of the high angle-of-attack (AOA) familiarization training, currently performed in the Schleicher ASK-21 glider, into the L-23. In order to build confidence in the ability of the L-23 to safely perform this mission requirement, USAFA requested the Test Pilot School (TPS) perform a limited evaluation of the high AOA flying qualities of the L-23.

SCOPE

Six test sorties were flown at the USAFA in Colorado Springs CO on 28 and 29 December, 1994. In addition, six test support sorties were flown in the ASK-21 for pilot familiarization.

OVERALL TEST OBJECTIVE

The overall test objective was to qualitatively evaluate the L-23 for mission suitability as a high performance training sailplane for use in the USAF Academy soaring program.

SPECIFIC TEST OBJECTIVES

The specific test objectives were to:

  1. Evaluate the high angle-of-attack flying qualities of the L-23.
  2. Qualitatively assess the stall characteristics of the L-23 within the flight manual center of gravity range.
  3. Assess the effectiveness of the flight manual stall recovery procedure and its ease of employment.
  4. Qualitatively assess the normal spin characteristics of the L-23 within the flight manual center of gravity range.
  5. Assess the effectiveness of the flight manual spin recovery procedure and its ease of employment.

TEST ITEM DESCRIPTION

The L-23 Super Blanik was produced by the Let Aircraft Factory in the former Czechoslovakian Republic. It is a derivative of the L-13 Blanik, first put in worldwide service in the early 1960's. The L-23 was a 2-place tandem cockpit all metal (with the exception of fabric covered control surfaces) sailplane of conventional design. It had high aspect ratio cantilever, shoulder-mounted wings with five degrees of forward sweep and a three degrees of dihedral. The elevator was mounted high on the vertical tail and mounted a conventional elevator and pitch trim tab for pitch control. The control system was a conventional three-axis, fully reversible, non-powered system actuated by center mounted control sticks and rudder pedals connected to a combination of control rods and control cables. The main landing gear was semi-retractable through actuation of a mechanical control lever in either cockpit and was fitted with a mechanical brake actuated by a floor-mounted control lever in each cockpit. The tail landing gear consisted of a full-swivelling shock-mounted wheel. The empty and flight center of gravity was behind the main wheel. Instrumentation consisted of basic flight instruments. An optional 15 kilogram seat pan/cushion was provided for replacement of the standard front seat cushion to accommodate light weight front crewmembers. The aircraft was certified by the Federal Aviation Administration (FAA) in the Utility category. For the purpose of this test the L-23 was considered to be production representative.

TEST & EVALUATION

All test sorties were flown with two crewmembers. Test center of gravity varied from 31-39 percent mean aerodynamic chord (MAC). Flight Manual limits for c.g. were 23 percent and 40 percent, forward and aft, respectively (Reference 1). Center of gravity variations were accomplished through change of crew pairings and seating locations using the test team plus one USAFA instructor pilot. The optional seat ballast was not used as it would have forced the c.g. farther forward than 31 percent. In addition, the aircraft was ballasted with 22.5 pounds of lead secured in the baggage compartment. This ballast was intended to simulate the impending installation of a crew breathing oxygen system for high altitude flight. According the Flight Manual, this ballast had no effect on c.g. location. No other form of c.g. control was available or used.

Test Center of Gravity Matrix

The following combination of crew pairings were used, with the resulting c.g. locations.

Front Seat CrewRear Seat CrewGross WeightC.G.
AldrichNuccio109231%
NuccioAldrich109235%
HarkinsNuccio103239%
NuccioHarkins103236.5%
AldrichHarkins105734%
Notes: Pilot weights w/parachute: Aldrich (195 lb), Nuccio (170 lb), Harkins (135 lb).

Classic high angle-of-attack (AOA) flight test techniques (FTT) were used. After a tow to 12,000 ft MSL (field elevation 6500 ft MSL) a Phase A stall was accomplished to characterize the stall and determine stall speed. On most flights Phase B and C stalls (left and right) were accomplished to determine departure susceptibility. Spin entries were accomplished, both left and right, using the Flight Manual entry procedure. Flight Manual recovery controls (basically NASA Standard controls--rudder opposite turn and stick aft until rotation stops, then neutralize all controls) were used as well as the ASK-21 recovery method (rudder opposite turn, pause, then stick forward). Aileron effects, both into and away from the spin were investigated.

TEST RESULTS

Stall

A one knot per second bleed rate was achieved with approximately 10 degrees of pitch attitude. Stall warning, in the form of light airframe buffet was evident at approximately 37 knots. Stall, characterized by mild g-break occurred repeatably at 32 knots. This agrees with the Flight Manual predicted stall speed. The L-23 exhibited a pronounced uncommanded left yaw about one to two knots above the stall speed. Yaw rate generated was approximately 60 degrees/second. This motion was consistent both with neutral aileron and right aileron inputs up to 2/3 right control stick deflection. The ensuing departure resulted in an extremely steep nose attitude and the beginning of an incipient spin and/or spiral depending on c.g. location. All Phase B and C stall inputs resulted in left departures unless exaggerated right aileron control input was maintained throughout the maneuver. The test aircraft should be ground tested for control rigging or lateral asymmetry anomalies (R-1). Stall recovery, using the Flight Manual procedure (move control stick forward) was effective. Altitude loss due to the stall was less than 200 feet to the left and less than 100 feet to the right.

Spins

All spin entry attempts at c.g. positions forward of 36 percent MAC resulted in self recovery after approximately one turn even with full pro-spin controls held. After the initial nose pitch down to 50-60 degrees and one turn, the nose pitched further down (approximately 70 degrees) and the aircraft accelerated into a steep spiral. Entries to the left, despite the initial nose "slice" described above, exhibited the same self-recovery as right entries; except that right entries required significant right aileron input to initiate the spin. During the incipient phase of a right spin attempt, the ailerons could be centered, but the aircraft quickly entered a spiral dive. Recovery from the spiral dive was quick and consistent using the technique of slightly unloading, rolling wings level, and pulling three to four g's to recover to a normal glide airspeed. With the c.g. as near the aft limit as possible given crew loadings, sustained spins were attainable. The initial tendency for a left nose slice remained, but after the first pitch down the attitude remained fairly constant (non-oscillatory) and a yaw rate of approximately 120 degrees/second developed. The spin could be characterized as extremely steep, fast, and smooth. Entry to the right spin required right aileron input as mentioned previously, but then resulted in a spin nearly identical in characteristic to the left spin. Altitude loss per turn of spin was approximately 400 feet, conforming to the Flight Manual prediction. The flight manual recovery was effective, resulting in recovery in approximately 3/4 turn after initiation of controls. The aircraft recovered in a nose low sideslip. Use of the ASK-21 flight manual procedure was slightly more effective, resulting in recovery in approximately 1/2 turn. This recovery left the aircraft in more coordinated flight with slightly less altitude loss. The ASK-21 spin recovery procedure should be considered for adoption for the L-23 (R-2). Aggravated aileron inputs during developed spins resulted in predictable changes to spinning motion in accordance with spin theory for a wing-loaded aircraft. Aileron into the spin resulted in acceleration of yaw rate and initial pitch down. Aileron against the spin resulted in slowing of the yaw rate and an initial pitch up. Neither aileron input significantly delayed recovery or increased altitude loss.

CONCLUSIONS AND RECOMMENDATIONS

The L-23 was found to be unsuitable for the mission of augmenting the USAFA spin training and familiarization program. Normal crew loadings result in a c.g. forward of that required to attain repeatable and consistent spin entries. Further, no factory authorized means exists to artificially manage c.g. location as it does with the ASK-21 tail weight kit. These limitations severely limit the ability of the L-23 to demonstrate fully developed spins using standard entry and recovery procedures. Also, the tendency of the test aircraft to develop uncommanded yaw at airspeeds slighty above the published stall speed results in an aircraft that is extremely susceptible to departure (Reference 5). This characteristic is undesirable in an airplane used for training low-experience pilots as it requires increased emphasis on airspeed control during critical phases of flight such as approach and landing. Unless ground testing determines a fix for this demonstrated nose slice problem the test team recommends increased aircrew training in slow flight and near stall flight characteristics with emphasis on maintaining airspeed during maneuvering close to the ground (R-3).

The following recommendations are presented:

R-1 The test aircraft should be ground tested for control rigging or lateral asymmetry anomalies.

R-2 The ASK-21 recovery procedure should be considered for adoption for the L-23.

R-3 The test team recommends increased aircrew training in slow flight and near stall flight characteristics with emphasis on maintaining airspeed during maneuvering close to the ground.

GARY L. ALDRICH, Lt Col, USAF
Chief Soaring Instructor Pilot

JEFFREY W. NUCCIO, Maj, USAF
Soaring Instructor Pilot

REFERENCES

  1. Sailplane Flight Manual, Let L-23 Super Blanik, Do-L 23. 1011.5, Kunovice, Czechoslovakia, June 12 1992.
  2. Test Management Phase Planning Guide, USAF Test Pilot School, Edwards AFB, CA, March 1993.
  3. Flying Qualities Phase Planning Guide, USAF Test Pilot School, Edwards AFB, CA, March 1993.
  4. Non-TPS Aircraft Qualitative Evaluation, USAF Test Pilot School Operating Instruction 51-9, Edwards AFB, CA, 22 January 1993.
  5. MIL-F-83691B (USAF), Military Specification Flight Test Demonstration Requirements for Departure Resistance and Post-Departure Characteristics of Piloted Airplanes, 6510 Test Wing, Edwards AFB, CA, 1 Mar 91.


EAA Chapter 1000 Home Page
E-Mail: Web Site Director Russ Erb at erbman@pobox.com

URL: http://www.eaa1000.av.org/fltrpts/blanik/blanik.htm
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 2 March 1997